The field of the invention is nuclear magnetic resonance (NMR) imaging and, more particularly, methods for removing unwanted spin resonance response signals from an NMR signal.
It is well known that nuclear magnetic resonance (NMR) signals produced by hydrogen nuclei (.sup.1 H) provide a useful tool for monitoring human metabolism. However, it is also well known that .sup.1 H NMR signals suffer from unwanted signal components such as those from uncoupled spin resonances, such as water and coupled-spin resonances, such as those from lipids. These undesired spin resonances are typically three to four orders of magnitude larger than the spectral peaks of interest. Further, these undesired spin resonances are positioned approximately at the same spectral position as the desired metabolite peaks, rendering the detection of the desired metabolite peaks virtually impossible by conventional NMR techniques. Accordingly, it is highly advantageous to provide a method for acquiring spin resonance responses from coupled hydrogen spins in metabolites in the presence of other in vivo human tissue components such as water and lipids.
Several NMR methods have been proposed which utilize a narrow bandwidth RF excitation pulse which is centered at the frequency of the offending spectral peak, and which suppresses the unwanted resonance peak in the total acquired NMR response spectrum. The most straightforward approach is the application of a long presaturating RF excitation pulse utilized to suppress the offending signal peak response (usually that of a water resonance) prior to receiving and processing the desired spectrum Another technique utilizes a series of 90.degree., RF excitation pulses with interleaved delays, to maneuver the undesired spin magnetization into a longitudinal direction, while the spin magnetization of the desired resonances are maneuvered into the transverse plane where they produce a detectable NMR signal. These techniques are limited because they also suppress desired spin resonant components which are at or near the frequency of the suppressed component, and they do not suppress other undesired spin resonant components at other frequencies.
Still other techniques are known which discriminate against the water resonance peak by taking advantage of the differences in the spin-lattice relaxation time T.sub.1 and the spin-spin relaxation time T.sub.2 between the undesired water spins and other chemical spins. Thus, long echo times can be effectively utilized to suppress the water peak in some tissues, while leaving other resonances, such as that of lactate, substantially unaffected. Many of the undesirable lipid resonances are also affected by this technique and are attenuated in the acquired NMR signal.
Other methods utilize an inverting pulse, having a delay equal to the null time constant (T.sub.null) of the undesired spin component (water), prior to readout. While suppressing the later resonance, these methods also partially suppress the desired metabolite resonance peaks and do not, in general, suppress other undesired (lipid) spin resonances.
Several existing methods suppress unwanted NMR response signals produced by uncoupled spin resonances by utilizing the scalar coupling which exists between adjacent atoms of the same molecule. The spins are nutated into the transverse plane and are acted upon by a sequence of RF excitation pulses and delays which cause the spin magnetization produced by the desired coupled atoms to evolve in a manner different from the manner in which spin magnetizations produced by the uncoupled spins evolve. Some such techniques, such as the Homonuclear Polarization Transfer technique, use a nonselective 90.degree. RF excitation pulse and a delay to invert the phase of all coupled spins having a particular coupling constant J. Such methods cannot, however, suppress lipid resonances which are coupled to one another. This disadvantage may be overcome with yet another method known as Homonuclear Double-Resonance Difference Spectroscopy, which allows retention of certain coupled peaks, such as the lactate resonance, while excluding certain other resonances, such as the lipid alkyl resonances. This method applies a selective 180.degree. RF excitation pulse which is centered on one of the lactate peak frequencies to only invert the phase of the lactate resonance peak, to which the first resonance peak is coupled. This occurs only if the frequency of the selective pulse is correctly set to within about 1 Hz, and an incorrect frequency will cause the original lactate, peak to be distorted in phase or amplitude, and may result in the desired signal components cancelling one another in the final NMR signal.
In a co-pending U.S. patent application Ser. No. 181,956 entitled "Method For Volume Localized Spectral Editing of NMR Signals Produced By Metabolites Containing Coupled Spins", a pulse sequence is described in which the signal components produced by the metabolite molecules of interest are amplitude modulated as a function of the time period, t.sub.1, between two of its RF excitation field pulses. By conducting two such pulse sequences with different time periods, and subtracting the resulting NMR signals, the desired signal components are produced while unmodulated signal components from water and lipids are suppressed.
In another co-pending U.S. patent application Ser. No. 215,979 entitled "Two-Dimensional Method For Spectral Editing of NMR Signals Produced By Metabolites Containing Coupled Spins", a technique is described in which a series of pulse sequences are run and a time period (t.sub.1) between two of its RF excitation field pulses is incremented through a range of values. An array of NMR data is acquired and a two-dimensional Fourier transform is performed to produce a contour map, or graph, of chemical shift spectral components versus the zero quantum modulation frequency components.
While these two prior methods are satisfactory for NMR spectroscopy, they are unacceptable for direct conversion to an NMR imaging method. Unfortunately, when these methods are used to produce images, the levels of both water and lipid suppression are not sufficient when frequency encoding is used in the imaging sequence, and the signal-to-noise ratio is marginal.